Food & Beverage

Using Supel™ QuE Z-Sep+ QuEChERS and an SLB®-5ms GC Column

Reduced Fat Interference with Analysis of Pesticides in Avocados

The health benefits of avocado are associated with its high content of healthy fat, fiber, vitamins and minerals. Avocado consumption in the United States has steadily grown, with sources of the fruit being both domestic and imported.1 The fat content of avocado is typically in the range of 10-15%, and this can pose a special analytical challenge when doing pesticide residue analysis. If fat is not removed from sample extracts, it can result in elevated detection limits and contamination of GC and LC systems.

QuEChERS (Quick, Easy, Cheap, Effective, Rugged, Safe) is a very popular method for the extraction and cleanup of fruit and vegetable samples for pesticide residue analysis.2-4 The extraction step uses acetonitrile and a salting out effect (with MgSO4). Cleanup is then accomplished using dispersive SPE (dSPE) followed by a centrifuge step prior to analysis. Sorbent choices for cleanup include:

• PSA (primary-secondary amine) – for removal of polar pigments, sugars, and organic acids

• Graphitized carbon black – for removal of chlorophyll and carotenoids

• C18 – for removal of lipids and non-polar components

• Zirconia – for removal of fatty components

Our Supel QuE Z-Sep+ material consist of both C18 and zirconia bonded to the same silica particles. The C18 binds fats through hydrophobic interaction, while the zirconia acts as a Lewis acid, attracting compounds with electron donating groups, such as the hydroxyl (-OH) groups in mono- and diglycerides.

ExperimentalTwo cleanup sorbents, Z-Sep+ and PSA/C18, were compared for their effectiveness to remove fats from avocado extracts prior to GC-MS/SIM analysis for pesticides. The extraction and cleanup procedures used are summarized in Table 1. Multiple replicates of both unspiked and spiked (20 ng/g with a mixture of hydrophobic and polar pesticides) avocado samples were processed. All extracts were then analyzed directly by GC-MS (SIM mode) using large volume injection (LVI). Quantitation was done using a calibration curve prepared with matrix-matched standards.

Table 1. Extraction and Cleanup Procedure

1. Place 3 g of homogenized avocado sample into a 50 mL centrifuge tube (Cat. No. 55248-U)

2. Add 25 mL acetonitrile, mix for 1 min

3. Add contents of Acetate Extraction Tube (Cat. No. 55234-U), mix for 1 min

4. Centrifuge samples for 5 min

5. Transfer 3 mL of step 4 supernatant into a Z-Sep+ Cleanup Tube (Cat. No. 55296-U), mix, centrifuge, then transfer 1 mL of the supernatant into a vial for GC analysis

6. Transfer 3 mL of step 4 supernatant into PSA/C18 Cleanup Tube (Cat. No. 55229-U), mix, centrifuge, then transfer 1 mL of the supernatant into a vial for GC analysis

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Figure 1. Avocado Extracts after Cleanup.

Figure 2. Amount of Matrix Residue Remaining

Results & Discussion

Matrix Removal Less color was observed in the extract cleaned with Z-Sep+ (Figure 1). This is an indicator that this sorbent removes more pigment than PSA/C18. Gravimetric analysis was also performed on several extracts to determine the weight of matrix residue remaining in the supernatants (Figure 2). This includes extracts where no cleanup was performed. As depicted, less unwanted matrix residue remains in the extract after cleanup with Z-Sep+ sorbent.

Food & Beverage

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©2012 Sigma-Aldrich Co. LLC. All rights reserved. SAFC, SIGMA-ALDRICH and SUPELCO are trademarks of Sigma-Aldrich Co. LLC, registered in the US and other countries. Solutions within is a trademark of Sigma-Aldrich Co. LLC. FLUKA is a trademark of Sigma-Aldrich GmbH, registered in the US and other countries. SLB is a registered trademark of Sigma-Aldrich Co. LLC. ENVI-Carb and Supel are trademarks of Sigma-Aldrich Co. LLC. FocusLiner is a trademark of SGE International Pty Ltd. Supelco brand products are sold through Sigma-Aldrich, Inc. Purchaser must determine the suitability of the product(s) for their particular use. Additional terms and conditions may apply. Please see product information on the Sigma-Aldrich website at www.sigmaaldrich.com and/or on the reverse side of the invoice or packing slip.

Visit our Food and Beverage/Pesticides resources, sigma-aldrich.com/food-pesticides

Featured and Related ProductsDescription Qty. Cat. No.Supel QuE QuEChERS ProductsAcetate Extraction Tube, 12 mL 50 55234-UCitrate Extraction Tube, 12 mL 50 55227-UCitrate/Sodium Bicarbonate Extraction Tube, 12 mL 50 55237-UZ-Sep+ Cleanup Tube, 12 mL 50 55296-UPSA/C18/ENVI-Carb™ Cleanup Tube, 12 mL 50 55286-UPSA/C18 Cleanup Tube, 12 mL 50 55229-UPSA/ENVI-Carb Cleanup Tube 1, 12 mL 50 55230-UPSA/ENVI-Carb Cleanup Tube 2, 12 mL 50 55233-UPSA Cleanup Tube, 12 mL 50 55228-UEmpty Centrifuge Tube, 50 mL 50 55248-USLB®-5ms Capillary GC Columns30 m x 0.25 mm I.D., 0.25 µm 1 28471-U30 m x 0.25 mm I.D., 0.50 µm 1 28473-U20 m x 0.18 mm I.D., 0.18 µm 1 28564-U20 m x 0.18 mm I.D., 0.36 µm 1 28576-UAnalytical SolventsAcetonitrile, for pesticide residue analysis 34481

Pesticide RecoveryReplicates of unspiked and spiked samples were analyzed so that recovery data could be calculated. The MS was operated in the SIM mode to achieve better sensitivity. The recoveries were better with Z-Sep+ for almost all pesticides, with greater reproducibility (Figure 3). Recovery data for several pesticides could not be determined in extracts after PSA/C18 cleanup due to the presence of high amounts of sample matrix.

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Analyte Recovery of Selected Pesticides from Avocado

PSA/C18 CleanupZ-Sep+ Cleanup

Figure 3. Analyte Recovery and Spread (n=3)

12 14 16 18 20 22 24Time (min)

PSA/C18 Cleanup

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Time (min)

Z-Sep+ Cleanup

column: SLB-5ms, 20 m x 0.18 mm I.D., 0.36 µm (28576-U) oven: 70 °C (2 min), 15 °C/min to 325 °C (5 min) inj. temp.: Programmed, 60 °C (0.28 min), 600 °C/min to 325 °C (5 min) detector: MS (SIM mode) carrier gas: helium, 1 mL/min constant injection: 10 µL LVI, PTV solvent vent, rapid injection speed; split vent flow: 100 mL/min (5 psi) until 0.28 min, 60 mL/min at 2.78 min liner: 4 mm I.D., split type, wool packed FocusLiner™ with single taper design

ChromatographyBelow are GC-MS (full scan) chromatograms of two extracts – one after cleanup with Z-Sep+ sorbent and one after cleanup with PSA/C18 sorbent (Figure 4). The Z-Sep+ chromatogram exhibits significantly less matrix interference. The Z-Sep+ sorbent is a great cleanup sorbent choice for fatty foods prior to pesticide analysis. It removes more unwanted matrix, and results in better analyte recoveries.

Figure 4. GC-MS Chromatograms (same y-axis)

Including products from

OXN_CZ_DP11545-513822 / T412132

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